The present invention relates to a control system for a plant, and more particularly to a control system controlling a plant with a response specifying type controller based on a sliding mode control theory which is one of robust control theories.
There has been known a sliding mode controller for controlling a plant according to a sliding mode control which is one of response specifying type controls (Japanese Patent Laid-open No. 2000-110636, for example). Specifically, the sliding mode controller shown in this publication controls an internal combustion engine. In the sliding mode control, it is possible to specify (change) a damping characteristic of a deviation between an output of the plant (controlled object) and a control target value. Therefore, such control is called as “response specifying type control”. Other than the sliding mode control, a back stepping control is also known as a response specifying type control. In the response specifying type control, a control input to a plant is calculated using a switching function which is defined as a linear function of a deviation between a control target value and an output of the plant, and a damping characteristic of the deviation can be changed by changing the switching function.
When controlling a throttle valve actuating device for actuating a throttle valve of an internal combustion engine, with the sliding mode controller, there is a following problem:
The sliding mode controller controls a throttle valve actuating device so that a detected throttle valve opening may coincide with a target opening. In the throttle valve actuating device that actuates a valve body of the throttle valve via reduction gears, a steady deviation with respect to the target opening arises due to backlash of the reduction gears. Accordingly, it takes a certain time period for the sliding mode controller to settle the steady deviation. The time period required for settling the steady deviation becomes longer particularly after a direction of change in the target opening or the throttle valve opening is reversed. Therefore, there is a tendency that a performance of the throttle valve opening following up the target opening becomes lower when the target opening changes slightly.